[Haskell-cafe] more on the "Beautiful Concurrency" function "withdraw" application ...

[Galchin Vasili]

module Main where
import Control.Concurrent.STM
import Control.Concurrent
import System.Random

type Account = TVar Int

transfer :: Account -> Account -> Int -> IO ()
transfer from to amount
      = atomically (do {deposit to amount;
                        withdraw from amount})

deposit :: Account -> Int -> STM ()
deposit acc amount = withdraw acc (- amount)

withdraw :: Account -> Int -> STM ()
withdraw acc amount
   = do { bal <- readTVar acc;
          writeTVar acc (bal - amount) }



When I try

withdraw (TVar 10) ..

I get "Not in scope: data constructor 'TVar'.  OK .. "Tvar" is a class and
not a Haskell data type ... so now what?

Vasya
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